A wide variety of gaskets are known for use in sealing applications. Porous expanded polytetrafluoroethylene (PTFE) is widely used today as a gasket material. As disclosed in U.S. Pat. No. 3,953,566 to Gore, this material has numerous properties making it highly desirable as a gasket. These properties include being readily compressible and conformable, being chemically resistant, having relatively high strength, and being far less prone to creep relaxation and loss of sealing pressure than non-expanded, non-porous PTFE alone.
Furthermore, gaskets made from biaxially or multiaxially expanded PTFE have improved sealing performance as compared to uniaxially expanded PTFE gaskets. For example, gaskets made from multiaxially expanded PTFE are resistant to creep relaxation and cold flow in multiple directions. The multi-directional tensile strength in multiaxially expanded PTFE gaskets provides circumferential and radial strength to the gasket and increases the cut through resistance of the gasket. Enhanced radial strength and cut through resistance provided by multiaxially expanded PTFE is achieved when the plane of expansion of the expanded PTFE is substantially parallel to the flange surface on which the gasket is installed.
In many sealing applications, the gasket is used to seal the junction between flanges, such as between pipes. In such applications, expanded PTFE is a desirable material for the gaskets because the expanded PTFE gasket can be placed between the flanges, and the flanges can then be pressed together with the application of force, such as by tightening of bolts. This application of force compresses the expanded PTFE. As the expanded PTFE is compressed, its initial pore volume is reduced, thus densifying the expanded PTFE. Particularly with metal-to-metal flanges, it is possible to apply sufficient force (or “stress”) to the flanges to fully densify the expanded PTFE. Thus, in at least part of the expanded PTFE gasket, the pore volume is reduced to substantially zero, such that a fluid contained within the pipes is prevented from leaking between the flanges by the densified, non-porous PTFE gasket, which seals the flanges.
In many applications, particularly when harsh chemicals are used which could readily break down the metal, or the metal could contaminate the chemical which is being transported or housed, it is common to use glass-lined steel, glass, or fiberglass reinforced plastic (“FRP”) piping and vessels. Because this equipment is often used with extremely harsh chemicals, there is great desire to use PTFE gaskets to seal the connecting flanges of this equipment because of the well known extraordinary chemical resistance of PTFE. Unfortunately, non-expanded, non-porous PTFE gaskets are generally not conformable enough to effectively seal this type of equipment. In the case of glass-lined steel flanges, although there is a relatively smooth finish, there is often a large amount of unevenness or lack of flatness associated with the flanges. This unevenness or lack of flatness requires the gasket to conform to large variations around the perimeter as well as between the internal and external diameter of the flange in order for an effective seal to be created. Thus, a non-expanded, non-porous PTFE gasket is not conformable enough to provide an adequate seal in many of these applications.
Because expanded PTFE is conformable, it would be desirable to use expanded PTFE to seal these commonly uneven flanges. Unfortunately, in many applications it is not possible to apply sufficient force to the flanges to create enough gasket stress to fully densify the expanded PTFE gasket to create an effective seal. For example, glass-lined steel piping flanges, glass flanges, or FRP piping flanges may deform, fracture, or break upon the application of a high amount of stress. Thus, in these applications, an expanded PTFE gasket may not be completely densified to reach a non-porous state, and therefore does not become leak proof, because the maximum stress that can be applied to the flanges without breaking them is not sufficient to densify the gasket. In some constructions where expanded PTFE gasket is not densified to a substantially non-porous state, leakage can occur through the residual porosity within the gasket. Often, this leakage is detected immediately after the installation of the gasket through either a “sniffing” technique or a “bubble test”. In the bubble test, a solution such as soapy water is applied to the gasketed flange and an internal air pressure is applied to the piping system or vessel. If a leak of a sufficient rate is present, bubbles will form in the soapy water solution. In some cases, a leak may exist but a rate small enough not to form a bubble. Where corrosive chemicals are being processed, the leak may persist for months or years and the corrosive chemicals can eventually leak through the gasket and attack the flange bolts or clamps resulting in a catastrophic failure of the flange.
U.S. Pat. No. 6,485,809, in the name of Minor et al., teaches a low stress to seal gasket construction comprising a multilayer, unitary gasket including at least one inner layer of expanded PTFE disposed between a first and second substantially air impermeable outer layer, and a substantially air impermeable region bridging the first and second substantially air impermeable layers. By “low stress to seal” is meant a gasket which provides a substantially air tight, or air impermeable, seal upon the application of a relatively low stress (i.e., a stress below that required to fully densify a porous expanded PTFE gasket, generally less than about 20,700 kPa (3000 psi)). This patent teaches gaskets which are stamped or cut from multilayered laminated sheets formed by wrapping layers around a mandrel, and subjecting gaskets to compressive treatment to compress a discreet portion and form an air impermeable region. While this patented construction may overcome many challenges in creating a low stress to seal gasket, there are limitations to the sizes of gaskets that can be produced when cutting gaskets from sheet goods. The largest size gasket that can be produced when cutting from sheet gasketing cannot be larger than the sheet size itself. Another concern with the manufacturing of such large size gaskets from sheet gasketing materials is the cost associated with producing such gaskets. For example, tooling costs for large size gaskets can be quite expensive and the manufacturing efficiencies of cutting gaskets from sheet stock can be relatively low especially with large diameter gaskets. When cutting gaskets from sheet stock, it is not uncommon to experience a sheet utilization yield of only 40% where the remaining 60% of the sheet is scrapped due to center drops, poor nesting of different size gaskets and unused corner sections.
U.S. Pat. No. 4,990,296 to Pitolaj teaches a method of welding together filled sintered PTFE components, wherein large diameter gaskets can be formed in sections by welding the ends of the sections together. This method, while perhaps suitable for sintered filled PTFE, would not be suitable for soft, porous expanded PTFE which would densify as a result of the applied heat and pressure at the welded joint. Densification would result in thinner, hard and non-conformable sections within the gasket which would less effectively seal fragile flanges such as glass lined steel and FRP flanges.
U.S. Pat. No. 5,964,465 to Mills et al. teaches a biaxially expanded PTFE form-in-place type gasket that is ideally suited for large size flanges. Form-in-place gaskets have the advantage of being able to be formed to any size flange without the limitations of gaskets cut from sheet stock such as low material utilization rates and expensive tooling costs. Form-in-place gaskets made in accordance with the teachings of Mills et al., comprised of biaxially expanded PTFE, may have additional advantages offered by the biaxially expanded PTFE such as chemical resistance, dimensional stability, and resistance to creep relaxation. However, as previously noted, since adequate gasket stress cannot be applied to densify the ePTFE, these gaskets cannot effectively seal glass lined steel and FRP flanges.
In PCT publication WO01/27501 A1 to Dove et al., a form-in-place gasket comprising an inner layer of expanded PTFE and substantially air impermeable outer layers that are bridged by a substantially impermeable region is taught. The substantially air impermeable outer layers and substantially air impermeable region are intended to prevent permeation through the expanded PTFE gasket material. The purpose of this gasket construction is to provide a tight seal at the low stresses where ePTFE alone can not be fully densified by preventing leakage through the porous ePTFE. However, gaskets constructed according to the teachings of WO 01/27501 are subject to a number of disadvantages. For example, outer air impermeable layers made of incompressible materials such as full density PTFE or densified expanded PTFE may increase the stiffness of the gasket, making it too rigid for a form-in-place gasket. It is desirable for form-in-place gaskets to be flexible so that they can be formed to the geometry of the flange.
Further, form-in-place gaskets comprising biaxially expanded PTFE are typically joined at the ends by skive-cutting the ends on a diagonal and overlapping the skive cut ends as taught in U.S. Pat. No. 5,964,465. Form-in-place gaskets constructed in accordance with PCT publication WO01/27501 A1 to Dove et al. having the outer impermeable layers, cannot be joined by overlapping the ends of the tape using the skive cutting technique without compromising the air impermeable nature of the material. When a skive cut is made through the outer air impermeable layers, porous expanded PTFE is exposed, providing a leak path through the gasket.
In U.S. Patent Publication No. 2003/0003290 A1 to Hisano et al., a sealing material in the form of a tape is taught which consists of laminated layers of porous expanded PTFE which are slit into strips having a height greater than the width, and wherein the laminated end faces on the long side of the laminated strip are in contact with the tightening surface. A plurality of the laminated strips may be joined together on the laminated surfaces of the laminate with tetrafluoroethylene-hexafluoropropylene copolymer or tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer film. It is further taught that at least one layer may be interposed within the laminate for preventing fluid penetration. In the form of a closed ring or gasket where the longitudinal beginning and end of the tape has been joined, the layers of expanded PTFE and the layer for preventing fluid penetration are vertically oriented when the gasket is installed on a flange surface. The layers intended to prevent fluid penetration in the radial direction may provide the gasket with low stress to seal capability by preventing leakage through the porous ePTFE. For gaskets made according to this method, the longitudinal strength of the expanded PTFE provides strength to the gasket in the circumferential direction when the gasket is installed on a flange surface. However, with the ePTFE layers laminated in the width direction, the transverse directional strength of the ePTFE is oriented in the vertical or “z” direction of the gasket. Therefore, little to no strength is provided to the gasket in the radial direction. Therefore, gaskets taught in U.S. Patent Publication No. 2003/0003290 A1 would be prone to cold flow in the width direction and lack dimensional stability. For gasketing applications involving glass lined steel flanges it is critical for the gasket material to be dimensionally stable to prevent fracture of the glass lining.
It would be desirable to provide a unitary, chemically resistant, dimensionally stable, high strength gasket material that can seal openings, especially glass-lined steel and FRP flanges, upon the application of a relatively low stress. Preferred gaskets made from this material are form-in-place gaskets, and it is further desirable that such a gasket can be installed using the common skive cutting techniques for overlapping the ends of the tape. Accordingly, it is the purpose of the present invention to provide an expanded PTFE tape that when in the form of a gasket provides a substantially air impermeable seal upon the application of a low stress, that is dimensionally stable and can be installed using the skive cutting overlap method.